Significant interests exist to detect and quantify specific metal ions in live tissues or animals, as metal ions play important roles in many cellular processes and functions including gene expression and signal transduction.
Trivalent cations have important biological properties and are directly involved in the cell function where there is a critical control of M3+ levels. For example, Cr3+ has direct impacts on the metabolism of carbohydrates, fats, proteins and nucleic acids by either activating certain enzymes or stabilizing proteins and nucleic acids. Chromium deficiency can increase the risk factors associated with diabetes and cardiovascular diseases. Al3+ could also have adverse effect on human's health, as excessive amount of Al3+ in the brain, is believed to cause neurodementia such as Parkinson's disease, Alzheimer's disease and dialysis encephalopathy. Fe3+ plays an indispensable role in many biochemical processes at the cellular level, and in the oxygen transport processes in all tissues in the form of hemoglobin. The deficiencies or excesses of Fe3+ can lead to a variety of diseases, such as Alzheimer's, Huntington's, and Parkinson's diseases. Thus, there is an urgent need to develop chemical sensors that are capable of detecting the presence of Cr3+, Al3+ and Fe3+ ions in biological samples.
Due to their paramagnetic nature, trivalent chromium (Cr3+) and iron (Fe3+) are among the most effective fluorescent quenchers, which makes it difficult to develop a fluorescence turn-on sensor. For this reason, very few sensors for Cr(III) and Fe (III) have been reported, and far fewer find application in cell imaging. In contrast Al3+ is diamagnetic, whose binding to sensors often enhance the fluorescence. Due to strong hydration of Al3+ in water, however, most reported dyes for Al3+ are required to be used in organic solvents or mixed solvents, with very few being suitable for Al3+ imaging applications. Recently, the study by Costero et al. reported a fluorescein derivative, whose fluorescence at 475 nm could be turned-on by Cr3+, Fe3+ and Al3+ in dry CH3CN (A. Barba-Bon, A. M. Costero, S. Gil, M. Parra, J. Soto, R. Martínez-Máñez and F. Sancenón. Chem. Commun., 2012). The presence of 4% of water in CH3CN, however, will quench the fluorescence of fluorescein complex with Cr3+ and Fe3+ ions. It remains a challenge to design a fluorescent sensor that not only can recognize but also differentiate the trivalent cations (Al3+, Cr3+ and Fe3+), especially in aqueous solution.
As the second most abundant transition-metal ion in the human body, the Zn2+ ion is a component of enzymes and proteins, and plays an important role in various biological processes. In order to discover the vital roles of Zn2+ in biological processes, there is growing demand for sensing Zn2+ in living systems. Although many fluorescent chemosensors for Zn2+ cation have been studied, few near-infrared (NIR) fluorescent zinc probes are available to give emission in the desired 700-900 nm range. An ideal Zn2+ probe requires not only NIR emission (to minimize autofluorescence) but also large Stokes shift (for improved signal detection). It is thus desirable to incorporate the ESIPT process into the sensing scheme. Achieving the ESIPT emission signals in the NIR region, however, remains an attractive and challenging task.